1. Field of the Invention
The present invention relates to apparatus, processes and solder pastes for soldering electronic components to a printed circuit board, and an assembly of the printed circuit board and electronic components obtained by way of soldering. More particularly, the present invention relates to a reflow soldering technology for soldering electronic components to a printed circuit board by applying a solder paste onto the printed circuit board, mounting the electronic components on the printed circuit board, and heating the printed circuit board and the electronic components.
2. Description of the Related Arts
There is conventionally known the art of electrically connecting and mechanically fixing surface mounted devices (SMDs) to a printed circuit board by way of reflow soldering in an assembly process of the surface mounted devices (SMD's) and printed circuit board.
In the reflow soldering process, SMDs are first mounted on the printed circuit board having pads on which a solder paste (also referred to as creamed solder) is preliminarily printed, with the lead terminals of the SMDs being aligned with the pads, and then the printed circuit board is introduced into a reflow apparatus (or heating apparatus).
The printed circuit board is transported within the reflow apparatus by means of a conveyor, and discharged therefrom after a predetermined time period. In the reflow apparatus, the printed circuit board is preheated to mitigate a thermal shock, then subjected to subsequent reflow heating (or main heating) to melt the solder, and after self-cooling (or forced cooling) the SMDs are bonded to the printed circuit board.
In this case, the lead terminals and bonding pads to be soldered are made of metals having high infrared reflectances, hence it is difficult to directly heat the solder paste up to a soldering temperature to achieve soldering. Therefore, heat conduction from the printed circuit board and SMDs is utilized to heat the solder paste up to the soldering temperature. Since the melting point of the solder is typically 183.degree. C., the soldering temperature is generally set to between about 210.degree. C. and about 230.degree. C., which is 30.degree. C. to 50.degree. C. higher than the melting point of the solder.
Such a conventional soldering apparatus (reflow soldering apparatus) incorporates infrared radiation heaters in a preheating zone and in a reflow zone (main heating zone) thereof, and is so designed to adjust the temperature profiles during the transportation of the printed circuit board by controlling the outputs of the infrared radiation heaters. See Japanese Unexamined Patent Publications No. 62-203669 (1987) and No. 1-254383 (1989).
In recent years, there has been an increasing need for mounting a large number of various SMDs on a printed circuit board. Therefore, apparatus and processes capable of reliably and efficiently soldering a large number of SMDs having different heat capacities to a printed circuit board are required. Further, such apparatus and processes are also required for a large-scale printed circuit board.
However, the conventional soldering processes and apparatus present the following problems:
(1) With low settings of the outputs of infrared radiation heaters, the temperature of solder joints does not reach a soldering temperature in regions where large-scale SMDs (having a high thermal capacity) are mounted, that is, the solder joints are subjected to insufficient heating. If an attempt is made to sufficiently heat the solder joints of the large-scale SMDs by increasing the outputs of the heaters in order to avoid such insufficient heating, the printed circuit board and small-scale SMDs with low heat capacities are overheated. This will result in a breakage or cracking of wiring conductors on the printed circuit board and in a damage or characteristic degradation of the small-scale SMDs. PA1 (2) In the case of a large-scale printed circuit board, it is difficult to uniformly heat many joint portions, hence to obtain satisfactory solder joints.